In recent years, large-scale integrated circuit (LSI) techniques have been greatly developed with remarkable improvements in the degree of integration, in particular. Following such improvement of the degree of integration and due to an increase of the IC chip size, the heat value generated in a package carrying such an IC chip thereon, is increased. Thus, the heat radiation property of a substrate material used as a package for a semiconductor device or the like has become important. An alumina sintered body, which has been widely used as an IC substrate, cannot cope with such an increase of the generated heat value of the IC chip, since alumina has only a small thermal conductivity and hence heat radiation property of an alumina sintered body is insufficient. Beryilia has been studied as a substitute for such an alumina sintered body since Beryllia has a higher thermal conductivity than alumina. However, beryllia is hard to handle due to its toxicity.
On the other hand, a sintered body of aluminum nitride (AlN) has been considered with interest as a material for a circuit board or a package for a semiconductor device, since aluminum nitride has an essentially superior in thermal conductivity and insulation ability. Additionally AlN is not toxic.
In general, an impurity content of an AlN sintered body is so reduced that an AlN sintered body with excellent thermal conductivity and electrical insulation ability is obtained. Such a sintered body is preferably used as an optical filter or the like in practice, since the same has also an excellent transparency. When the AlN sintered body is used as a package or the like, however, the transmission of ultraviolet rays leads to malfunction of the IC, resulting in a significant problem.
In order to cope with this problem, colored AlN sintered bodies have been studied in various ways, and several successful cases have been reported.
For example, Japanese Patent Laying-Open Gazette No. 63-233079 (1988) discloses a black aluminum nitride sintered body and a method of preparing the same. According to this literature, a specific amount of calcium tungstate powder and/or calcium molybdate powder is added to and mixed with AlN powder, and the mixture is molded and then fired in a non-oxidizing atmosphere, to obtain a black AlN sintered body.
On the other hand, Japanese Patent Laying-Open Gazette No. 63-310772 (1988) discloses a black AlN sintered body which is obtained by sintering AlN powder with an additive of metal molybdenum or a molybdenum compound. However, the aluminum nitride sintered bodies disclosed in the aforementioned gazettes have only low thermal conductivities of about 100 W/m.multidot.K.
Since AlN easily incorporates an impurity such as oxygen, high-purity AlN powder is generally employed in order to improve the thermal conductivity of an AlN sintered body. If any one of the aforementioned materials is added to AlN in order to color the AlN sintered body, its thermal conductivity is reduced. Thus, it is impossible to obtain a colored AlN sintered body which also has a high thermal conductivity.
A high thermal conductivity is required for a substrate material for carrying an IC chip having a high output, for improving the heat radiation property, in order to prevent the a malfunction of the IC due to a temperature rise of the IC chip. At the same time, the substrate must be colored in order to prevent the an IC malfunction due to transmission of ultraviolet rays. According to the prior art, however, it has been difficult to provide a ceramic substrate which can satisfy the two requirements for the substrate material for an IC chip, as hereinabove described.